EAF dust is a by-product of steel scrap smelting and is considered a "hazardous material" due to the presence of hazardous metals therein (e.g., heavy metals such as lead, zinc, cadmium and the like). In this regard, EAF dust that is obtained from processing ferrous steel scrap will have a "hazardous material" rating (i.e., K061 per U.S. Environmental Protection Agency Standards), due to the presence of lead, cadmium and/or zinc in addition to iron. A major source of K061-rated EAF dust is steel scrap that is mixed or coated with the elements that render dust hazardous. Salvaged ships, structural steel, galvanized steel and primarily automobile scrap are the major sources of feed to the smelting furnaces. Automobile scrap is, however, also a source of constituents used in plastic components found in the construction of automobiles. Thus, in addition to heavy metal contaminants, EAF dust could contain typically a meaningful amount of chlorine, usually on the order of about 2 wt. %, which originates from road salt that contaminates most scrap automobiles used as feed stock for the electric arc steel smelting furnace.
Broadly, the present invention involves the major recovery of the primary metals (e.g., iron, zinc, cadmium and lead) from the slag producing constituents in the EAF dust by bringing the dust into contact with a plasma arc of a DC sealed atmosphere plasma arc furnace so as to volatilize (vaporize) those metals having a vaporization point less than the lowest temperature of exposure within the furnace, and partition from the melt those constituents with a vaporization point higher than the lowest temperature of exposure within the furnace. Any halides, for example, chlorine, that may be present in the feed material, will vaporize such that the halide (chloride) ions are brought into contact with an alkali metal compound to form alkali metal chlorides and thereby to prevent substantially the formation of heavy metal chlorides.
In particularly preferred forms, the EAF dust is subjected sequentially to two high temperature zones within the furnace. The first high temperature zone is established immediately below the electrode in the plasma arc, while the second high temperature zone is in the form of a "slag" which circulates by induction and natural arc stirring relative to the first zone. The EAF dust and carbonaceous reductant is fed into the furnace through a hollow electrode member cocurrently with the plasma gas. As such, the EAF dust first encounter the ultrahigh temperature (e.g., greater than 3,000.degree. F.) immediately below the electrode in the first high temperature (plasma) zone. The EAF dust encountering such high temperature will decompose rapidly so as to vaporize at least a portion of the hazardous metals contained therein.
The second high temperature zone is the gas phase above the molten slag. It will be at a temperature almost as hot as the slag--that is, at least about 3,000.degree. F. Chemical decomposition reactions and metals vaporization are completed in the hot gas of the second high temperature zone. Furthermore, controlled additions of an oxidizing gas, such as oxygen, air, or steam, may be introduced into the gas space within the furnace through a cooled lance inserted through an auxiliary port in the roof of the furnace to partially combust any residual or excess carbon. The preferred embodiment utilizes a metered quantity of oxygen for this purpose so as to minimize the volume of off-gas produced. The produced off-gas, however, is brought into contact with an alkali metal compound (e.g., either in the furnace itself or in the off-gas discharge line) so as to covert the free chlorides therein to alkali metal chlorides. As such, free chlorides in the off-gas are prevented from reacting with the hazardous heavy metal vapors so that relatively pure heavy metals may be recovered (e.g., heavy metals and/or metal oxides substantially uncontaminated by metal chlorides). Furthermore, the furnace may be operated in an oxidizing or reducing condition as required to maximize the recovery of the metal constituents in the EAF dust feed.
The slag is essentially amorphous, and when cooled is non-leachable. Iron is recovered as iron metal for reuse.
These and other aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.